Using the Tertiary Volcanic and Gravelly Sedimentologic Record to Test the Timing and Evolution of the Chocolate Mountains Anticlinorium, SE California

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。巧克力山背斜的起源及其对美国西南部第三纪构造和构造史的意义仍然是个谜。这些中生代至第三纪的岩石不连续地暴露在整个加利福尼亚东南部，可能来自于俯冲带或弧前盆地环境，最初位于古太平洋以西约300 - 400公里处，但它们现在形成了圆顶状结构，通常被称为CMA。目的是确定CMA最初是在古法拉龙板块（位于太平洋和北美板块之间）的古近纪低角度俯冲期间形成的，还是在美国西南部的中第三纪伸展期发掘过程中形成的。此外，研究人员将评估加利福尼亚东部剪切带的早期走滑断层（现代圣安德烈亚斯断层的前身）在背斜发育过程中可能发挥的潜在作用。为了解决这些问题，将开发一系列地质年代学限制的时间片，这些时间片描绘了与CMA形成有关的岩石和沉积物的演化。本研究的预期结果是为巧克力山的发育及其对美国西南部形成和演化的意义提供一个增强模型。CMA模型时间片发育的关键是新近纪熊峡谷砾岩，该砾岩可细分为三个不整合结合单元。值得注意的是，初步工作表明，熊峡谷的中下游段位于背斜的两侧，并记录了背斜的增量增长，最后一次增量是由黑山~ 900万~ 1300万年前的层间玄武岩的倾斜记录的。首席研究员和大量学生将开始他们的工作，追踪约300平方公里的熊峡谷砾岩的下、中、上部成员的不整合程度。他们的工作将包括对层理类型、碎屑和粒度分布、沉积构造和古洋流方向的详细描述。此外，激光烧蚀U-Pb锆石和选定的Ar-Ar年代学将确定熊峡谷下所有主要的中生代和第三纪火山单元，以及新近纪砾岩中的碎屑。在中生代基底和第三纪火山岩之间的始新世/渐新世沉积岩将作进一步的分析。通过对过去6000万年来积累的褶皱生长和断层滑动的顺序去除，PI和学生们将能够评估背斜的增量生长，以及俯冲、伸展挖掘和前圣安德烈斯走滑断层在古近纪（~ 6550万~ ~ 3390万年前）和新近纪（~ 2300万~ ~ 260万年前）背斜的发展和加利福尼亚东南部景观演变中可能发挥的潜在作用。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).The origin of the Chocolate Mountains anticlinorium (CMA) and its significance to the Tertiary structural and tectonic history of the southwestern United States remains enigmatic. Discontinuously exposed throughout southeastern California, these Mesozoic to Tertiary age rocks may have been derived from a subduction zone or forearc basin setting located initially ~300 - ~400 kilometers to the west within the ancient Pacific Ocean, but they now form dome-like structures commonly referred to as the CMA. The goal is to determine whether or not the CMA formed initially during Paleogene low-angle subduction of the ancient Farallon plate, which lay between the Pacific and North American plates, or somewhat later during middle Tertiary extensional exhumation of the southwestern United States. In addition, the investigators will evaluate the potential role that early strike-slip faults of the Eastern California Shear Zone, a precursor to the modern day San Andreas fault, may have played in the development of the anticlinorium. To address these issues, a series of geochronologically constrained time slices that portray the evolution of the rocks and sediments involved in the formation of the CMA will be developed. The expected outcome of this research is an enhanced model for the development of the Chocolate Mountains and their significance to the formation and evolution of the southwestern U.S.Key to the development of the CMA model time slices is the Neogene Bear Canyon conglomerate, which can be subdivided into three unconformably bound units. Significantly, preliminary work indicates that the lower and middle members of the Bear Canyon lie on both limbs of the anticlinorium and record its incremental growth with the last increment recorded by the tilting of the interstratified ~9-13 million year old basalts of Black Mountain. The lead investigator and a large number of students will commence their work by tracing out the extent of the unconformities bounding the lower, middle, and upper members of the Bear Canyon conglomerate over ~300 km2. Their work will include detailed characterization of bedding types, clast and grain-size distributions, and sedimentary structures and paleocurrent directions. In addition, laser-ablation U-Pb zircon and selected Ar-Ar geochronological ages will be determined for all key Mesozoic and Tertiary volcanic units underlying the Bear Canyon, as well as for clasts included in the Neogene conglomerates. Additional analyses will be made on Eocene(?)/Oligocene(?) sedimentary rocks lying between Mesozoic basement and Tertiary volcanic rocks. By sequentially removing fold growth and fault slip that has accumulated over the last ~60 million years, the PI and students will be able to assess the incremental growth of the anticlinorium and the potential role that subduction, extensional exhumation, and pre-San Andreas strike-slip faulting may have played in its development and the evolving landscape of southeastern California during the Paleogene (~65.5 to ~33.9 million years ago) and Neogene (~23 to ~2.6 million years ago).